The advent of a new round of stringent emissions legislation in Europe and North America is driving the implementation of new exhaust after-treatment systems, particularly for lean-burn technologies such as compression-ignition (diesel) engines, and stratified-charge spark-ignited engines (usually with direct injection) that are operating under lean and ultra-lean conditions. Lean-burn engines exhibit high levels of nitrogen oxide (NOx) emissions that are difficult to treat in oxygen-rich exhaust environments characteristic of lean-burn combustion. Exhaust after-treatment technologies are currently being developed that will treat NOx under these conditions. One of these technologies comprises a catalyst that facilitates the reactions of ammonia (NH3) with the exhaust nitrogen oxides (NOx) to produce nitrogen (N2) and water (H2O). This technology is referred to as Selective Catalytic Reduction (SCR).
Ammonia is difficult to handle in its pure form in the automotive environment. Therefore, it is customary with these systems to use a liquid aqueous urea solution, typically at a 32% concentration of urea solution (CO(NH2)2). The solution is referred to as AUS-32, and is also known under its commercial name of AdBlue. The urea solution is delivered to the hot exhaust stream and is transformed into ammonia in the exhaust after undergoing thermolysis, or thermal decomposition, into ammonia and isocyanic acid (HNCO). The isocyanic acid then undergoes a hydrolysis with the water present in the exhaust and is transformed into ammonia and carbon dioxide (CO2). The ammonia resulting from the thermolysis and the hydrolysis then undergoes a catalyzed reaction with the nitrogen oxides as described previously.
In today's production systems, the RDU is typically mounted under the body of the vehicle, in a downstream location on the exhaust line. This results in relatively low temperatures at the SCR catalyst, longer light-off times, and low conversion efficiency of the NOx. The lower exhaust temperatures (lower enthalpy) also inhibit the thermal decomposition of the urea thermolysis reaction, or in the case of the thermolysis HNCO byproduct, the low temperatures also inhibit the hydrolysis reaction. The result is the presence of excessive urea and/or HNCO at the SCR catalyst and an insufficient quantity of ammonia to participate in the NOx reduction reactions. A good example of this situation was presented in SAE 2007-01-1582: “Laboratory and Engine Study of Urea-Related Deposits in Diesel Urea-SCR After-Treatment Systems”. Engine dynamometer data from this study shows that at exhaust temperatures below 300° C., a measurable proportion of the injected urea remains untransformed into either HNCO or NH3.
There are also activities in the industry examining the potential of alternative reducing agents. Some of these agents (e.g., Guanidinium Formate) exhibit higher decomposition temperatures than those of urea. In order for these alternatives to be viable, they require preheating, typically in a dedicated reformer located in a bypass flow passage off the main exhaust. A description of one such approach is provided in in SAE 2012-01-1078, “Development of a 3rd Generation SCR NH3-Direct Dosing System for Highly Efficient DeNOx”. During the startup phase, these reformer concepts typically rely on electrical heating of the bypass gas flow and the use of hydrolysis reaction catalysts to ensure the proper conditions for transformation of the carriers into ammonia.
With reference to FIG. 1, a conventional RDU 10 is shown, generally indicated at 10, having a fluid injector 12. The injector 12 is for use in delivering fluid, such as urea solution and employs an inductive coil heater 13 with the goal to transfer heat from the coil heater 13 to an inlet tube 14 of the injector 10 and to the fluid. With reference to FIG. 2, however, due to limitations imposed by the installation of a port fuel injector in a cylinder head or intake manifold, the coil heater 13 does not extend down fully to the tip or exit 16 of the injector 12. The result is a magnetic flux path, indicated by arrows A in FIG. 2, that is limited in extent and which induces heating that terminates 6-8 mm above the metering point or exit 16 of the injector 12. The result is an unheated volume V of fluid of 213 mm3 that does not benefit from a direct heating path to the inductive heat source (coil 13). This volume V needs to be evacuated before heated fluid is able to be ejected—at a flow rate of 5.2 mg/s (a typical flow rate during a vehicle cold start on emissions test cycles), it would theoretically require a minimum of 45 seconds to remove this unheated fluid. In urea injection applications where cold start activity is required, this delay reduces the effectiveness of the system to start reducing the engine-out NOx emissions.
Thus, there is a need to directly heat a reducing agent in an injector closer to the metering point to ensure a more efficient heat transfer and produce the desired reducing agent temperature so as to reduce the time required to remove unheated reducing agent.